The present invention relates to low noise cables with operating temperatures of the order of 250.degree. C.
Such cables are screened. They comprise a conductive core covered with a PTFE dielectric, a conductive layer arranged on the dielectric and covered with a screen, and a protective external insulating sheath covering the screening. The conductive layer combined with the screening provides improved protection, particularly against low frequencies, for which the cable is said to be anti-noise.
This screening is generally constituted by a braid of conductive wires, particularly wires of bare, nickel-plated or silver-plated copper. For its part, the conductive layer is constituted by a conductive tape or preferably by a conductive varnish, the latter providing better noise immunity to the cable than the tape.
Conductive varnishes are coatings comprising a PTFE based polymer filled with fine conductive particles; they therefore adhere very strongly to the dielectric and provide the desired low level of noise.
However, such conductive coatings are difficult to remove locally at and in the immediate vicinity of the ends of the cables which are provided with connectors. Such removal makes it possible to avoid degradation of the coating at these locations, which degradation is due to vibrations and rubbing that may cause the conductive particles of the coating to become detached and move, thereby causing a short-circuit between the core and the screening in the connectors.
These conductive PTFE based coatings are insoluble in most common solvents. They are removed locally essentially by mechanical means, particularly by scraping or abrasion. This operation is lengthy and difficult, but above all the desired removal is not perfect and may therefore still lead to the risks indicated above.